docs: def activate_clauses(self, beg):
          """
              This method is used for activating the clauses that belong
              to optimization levels up to the newly computed level. It
              also reactivates previously deactivated clauses (see
              :func:`process_sels` and :func:`process_sums` for
              details).
          """
  
          end = min(self.levl + 1, len(self.blop))
  
          for l in range(beg, end):
              for sel in self.wstr[self.blop[l]]:
                  if sel in self.bckp_set:
                      self.sels.append(sel)
                  else:
                      self.sums.append(sel)
  
          # updating set of selectors
          self.sels_set = set(self.sels)
  
          return end